ORIGINAL RESEARCH
published: 01 September 2015
doi: 10.3389/feart.2015.00044
Is the Neoproterozoic oxygen burst a
supercontinent legacy?
Melina Macouin 1*, Damien Roques 1 , Sonia Rousse 1 , Jérôme Ganne 1 , Yoann Denèle 1
and Ricardo I. F. Trindade 2
1
Géosciences Environnement Toulouse, Centre National de Recherche Scientifique, UMR 5563, UR234, Université de
Toulouse, Toulouse, France, 2 Departamento de Geofísica, Instituto de Astronomia, Geofísica e Ciências Atmosféricas,
Universidade de São Paulo, São Paulo, Brazil
Edited by:
Eric Font,
University of Lisbon, Portugal
Reviewed by:
Alexandra Abrajevitch,
Institute of Tectonics and Geophysics,
Russia
Qingsong Liu,
Chinese Academy of Sciences, China
*Correspondence:
Melina Macouin,
Géosciences Environnement
Toulouse, Centre National de
Recherche Scientifique, UMR 5563,
Université de Toulouse, 14 avenue E.
Belin 31400 Toulouse, France
[email protected]
Specialty section:
This article was submitted to
Geomagnetism and Paleomagnetism,
a section of the journal
Frontiers in Earth Science
Received: 27 January 2015
Accepted: 24 July 2015
Published: 01 September 2015
Citation:
Macouin M, Roques D, Rousse S,
Ganne J, Denèle Y and Trindade RIF
(2015) Is the Neoproterozoic oxygen
burst a supercontinent legacy?
Front. Earth Sci. 3:44.
doi: 10.3389/feart.2015.00044
The Neoproterozoic (1000–542 mol.yr ago) witnessed the dawn of Earth as we know
it with modern-style plate tectonics, high levels of O2 in atmosphere and oceans
and a thriving fauna. Yet, the processes leading to the fully oxygenation of the
external envelopes, its exact timing and its link with the inner workings of the planet
remain poorly understood. In some ways, it is a “chicken and egg” question: did the
Neoproterozoic Oxygenation Event (NOE) cause life blooming, low-latitudes glaciations,
and perturbations in geochemical cycles or is it a consequence of these phenomena?
Here, we suggest that the NOE may have been triggered by multi-million years oxic
volcanic emissions along a protracted period at the end of the Neoproterozoic when
continents were assembled in the Rodinia supercontinent. We report a very oxidized
magma source at the upper mantle beneath a ring of subducting margins around Rodinia,
and detail here the evidence at the margin of the Arabian shield. We investigate the
780 Ma Biotite and Pink granites and associated rocks of the Socotra Island with rock
magnetic and petrographic methods. Magnetic susceptibility and isothermal remanent
magnetization acquisitions show that, in these granites, both magnetite and hematite are
present. Hematite subdivides magnetite grains into small grains. Magnetite and hematite
are found to be primary, and formed at the early magmatic evolution of the granite at very
high oxygen fugacity. Massive degassing of these oxidized magmas would reduce the
sink for oxygen, and consequently contribute to its rise in the atmosphere with a net O2
flux of at least 2.25 × 107 Tmol. Our conceptual model provides a deep Earth link to the
NOE and implies the oxygenation burst has occurred earlier than previously envisaged,
paving the way for later changes in the outer envelopes of the planet epitomized on the
extreme Neoproterozoic glaciations and the appearance of the first animals.
Keywords: rock magnetism, Neoproterozoic oxygenation event, hematite-magnetite buffer, Rodinia, Socotra
Introduction
The Neoproterozoic Oxygenation Event (NOE) (Och and Shields-Zhou, 2012) (Figure 1A) marks
a turning point in the early Earth’s history after which atmospheric free oxygen has reached
the Present Atmospheric Level (PAL) as indicated by several different geochemical proxies (e.g.,
Canfield et al., 2007; Och and Shields-Zhou, 2012; Sahoo et al., 2012; Reinhard et al., 2013; Lyons
et al., 2014). But the mechanism leading to the full oxygenation of Earth’s external envelopes, its
exact timing (Baldwin et al., 2013) and its link with the inner workings of the planet remain poorly
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Is the Neoproterozoic oxygen burst a supercontinent legacy?
FIGURE 1 | (A) Evolution of Earth’s atmospheric oxygen content between 3.5 Ga and present (from Lyons et al., 2014). PO2 is the atmospheric partial
pressure of O2 and PAL is the present atmospheric level. The green and blue curves depict a two-step evolution of atmospheric evolution (Kump, 2008)
and a recent model (Lyons et al., 2014), respectively. In yellow are the classical time windows for the GOE (Great Oxydation Event) and NOE
(Neoproterozoic Oxydation Event). (B) Model of Cryogenian Oxygen cycle and subduction zones in the frame of Rodinia. Green zones highlight the
suspected hot and oxidized magma zone (OxMZ) under subductions.
volume the large magmatic activity of collision and dispersal of
continental fragments, well-preserved in the zircon record for
this time period (Hawkesworth et al., 2013), thus corresponding
to a peak in volcanic degassing between 850 and 750 millions of
years ago.
Here, we explore the role of subductions and associated
volcanic degassing on the redox state of the upper mantle,
atmosphere and oceans at Rodinian times by investigating rock
magnetic properties of the 840–780 Ma old rocks of the Socotra
Island.
understood. In some ways, it is a “chicken and egg” problem: did
the NOE cause life blooming (Och and Shields-Zhou, 2012), lowlatitude glaciations and perturbations in geochemical cycles or
is it a consequence of these phenomena (Lenton et al., 2014)?
But what if the Neoproterozoic oxygen burst was in fact a
supercontinent legacy? Some authors have evoked the possible
role of changes in global geodynamics on the increase of the
oxygen content in atmosphere and oceans (Kump et al., 2001;
Gaillard et al., 2011 and see a review in Kasting, 2013) but the
topic remains poorly explored.
Special geodynamical conditions prevailed during most of
the Neoproterozoic. By the time the continents were assembled
for the first time into a supercontinent the size of Pangea
(Hawkesworth et al., 2013). The Rodinia supercontinent (1000–
720 Ma) was the most long-lived supercontinent in Earth’s history
(Ernst et al., 2013; Li et al., 2013) (Figure 2). It was surrounded
by subduction zones (Li et al., 2013) whose relics are now
sparsely dispersed around the world as fossil magmatic arcs and
ophiolites. Subduction-related magmatism likely surpassed in
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A New Model for the Oxygenation of the
Neoproterozoic Atmosphere?
Nowadays, and probably, since the Archean, the redox state of
the mantle is considered to be around the Fayalite–Magnetite–
Quartz (FMQ) buffer as exemplified by modern MORB’s fO2 values (1 FMQ +0.5) (Frost and McCammon, 2008). Deeper
into the mantle, spinel peridotites display a larger range
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Is the Neoproterozoic oxygen burst a supercontinent legacy?
and Claire, 2005; Kasting, 2013) (Figure 1B). Today, with a total
emission of 1.5 Tmol.yr−1 , the volcanic sink for O2 is estimated
to consume around 25% of the O2 generated by photosynthesis
and subsequent organic carbon burial (Kump et al., 2001).
Below we detail the petrographic and magnetic evidence for
these Neoproterozoic highly oxidized magma in Socotra Island
(Yemen).
Geological Setting
In order to test for the oxidation state of magmas across
subduction zones surrounding Rodinia, and investigate their
eventual role on the rise of atmospheric oxygen, we studied
the Neoproterozoic basement outcropping at the Socotra Island
(Yemen) in the Arabic peninsula. The Socotra Island belongs
to the southern rifted margin of the Gulf of Aden (Figure 3),
extending the tip of Somalia to the ENE (Figure 3). The island
is about 135 km from east to west and 45 km wide from north to
south and lies about 250 km of Somalian coast (longitude: 53′ 19◦
and 54′ 33◦ E; latitude: 12′ 18◦ and 12′ 42◦ N). Most of the island
is covered by Mesozoic to Cenozoic pre- to syn-rift sedimentary
rock successions affected by tilted-block tectonics linked to the
Gulf of Aden opening (Leroy et al., 2012; Ahmed et al., 2014).
Three main basement outcrops can be observed in the island
(Denéle et al., 2012) (Figure 3), which corresponds to three
basement highs located at the head of tilted blocks. To the west
the Qalansya and Sherubrub areas represents respectively, 65 and
50 km2 of basement outcrop and to the east the Mont-Haggier
basement outcrop represents 580 km2 .
The basement of the Socotra Island displays a variety of
plutonic, volcanic and metamorphic rocks formed in an active
margin between 840 and 780 Ma (Denéle et al., 2012). A
voluminous highly potassic calc-alkaline granitic magmatism,
comprising the Biotite granite and the Pink Haggier granite
plutons, were emplaced in an Andean-type arc setting at 790
Ma (Denéle et al., 2012). Slightly before, mafic to intermediate
intrusions as vertical sheets, a kilometer-scale gabbro laccolith,
mafic dike swarms and lavas with a depleted arc signature
were emplaced in the upper crust of the arc. Together,
this assemblage represents a long-lived subduction-associated
magmatism spanning up to 50 Ma.
FIGURE 2 | Rodinia paleogeography model around 780 Ma (adapted
and modified from Li et al. 2013 and using Paleomac software, Cogne,
2003). In red is the extension of presumed subductions around the
supercontinent. Green zones highlight the suspected hot and oxidized magma
zone under subductions. Green dots represent indian (ind) and arabic (soc)
highly oxidized rocks related to subductions. Craton/terrane names
abbreviations: IND, India; A, Amazonia; Si, Siberia; SC, South China; NA,
Northern Australia; EA, East Antarctica; L, Laurentia; K, Kalahari; R, Rio de
Plata; C, Congo; B, Baltica; WA, West-Africa; NC, North China.
of fO2 -values between 1FMQ −2 and +2. Given the long
lifetime of Rodinia, the insulation effect due to the presence
of a supercontinent must have been important (Grigne and
Labrosse, 2001), increasing the temperature of the mantle
beneath the continent (Lenardic et al., 2011). Furthermore,
when a supercontinent is surrounded by subduction margins,
this heating effect is enhanced since subducting plates prevent
lateral thermal mixing of the mantle as shown by numerical
and analogical experiments (Lenardic et al., 2011). Another
consequence of the global temperature increase below the
supercontinent is a possible enhancement of the large-scale
convection by the reduced viscosity of the mantle, while smallscale convection will be almost absent preventing efficient mixing
(Samuel and King, 2014). This would result in a hot and poorly
mixed mantle beneath Rodinia. For the subduction zones, this
would mean an increase in the melting of the oxidized rocks of
the slab, resulting in a large zone of oxidized magmas, possibly
further enhanced by thick arcs (Chiaradia, 2014), around the
edges of Rodinia.
A large reservoir of oxidized magma beneath the fringe of
Rodinia probably resulted in large emissions of oxidized volcanic
gases (Burgisser and Scaillet, 2007) into the ocean-atmosphere
system with an impact on the balance between sources and sinks
for the free oxygen in the atmosphere. Indeed, the oxidation
of reduced volcanic gases is one of the major sinks for various
atmospheric oxidants, notably oxygen (Kump et al., 2001; Catling
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Methods
Neoproterozoic basement rocks of Socotra were sampled with a
gasoline-powered drill during a field trip realized in 2009–2010.
The Biotite and Pink granites and the Haggier layered gabbro
were sampled in the Haggier Mount domain. The geological map
of Socotra shows the location of the analyzed samples.
Forty-nine samples were selected for magnetic mineralogy
studies, 25 from the Biotite granite and others from the Pink
Haggier granite, the Layered Gabbros and from Mafic to
Intermediate Plutonic Sheets. Eight thin sections were analyzed
with scanning electron microscope (SEM) and reflected light
microscopy (Nikon Eclipse LV100POL). SEM observations
were performed with a JEOL instrument (JSM-6360LV) at the
GET laboratory (Toulouse, France) operating at 20 kV with
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Is the Neoproterozoic oxygen burst a supercontinent legacy?
FIGURE 3 | Geological map of Socotra (modified from Denéle et al., 2012), locating the different samples accross the Neoproterozoic basement.
magnetostatic interactions. For each formation, 80–100 FORCs
were measured with an averaging time of 100 ms and a waiting
time before successive measurements of 1 s. FORC diagrams
were processed using the FORCinel software (Harrison and
Feinberg, 2008) with an average smoothing factor (SF) of four
(see Roberts et al., 2000) after correction of the paramagnetic
slope. Paleogeographic map was drawn with paleomac software
(Cogne, 2003).
an Electron Dispersive System (EDS) that allows a punctual
qualitative characterization of the chemical composition.
Magnetic measurements consisted of thermomagnetic
curves, isothermal remanent magnetization (IRM) acquisition
curves, hysteresis loops, and first-order reversal curves (FORC)
diagrams. Temperature dependent magnetic susceptibility
curves from room temperature up to 700◦ C were obtained with
a KLY2-CS2 (AGICO) at the GET laboratory on 12 powdered
samples. IRM acquisitions (Kruiver et al., 2001; Heslop et al.,
2002), hysteresis loops and first-order reversal curves (FORC)
diagrams (Roberts et al., 2000; Harrison and Feinberg, 2008)
were made using a Princeton Measurements Corporation
Vibrating Sample Magnetometer (VSM: Micromag Model
3900) at the IPGP laboratory. IRM acquisition curves were
performed non-linearly (with 150 points) up to +1.5 T. IRM
acquisition curves were analyzed using the excel spreadsheet of
Kruiver et al. (2001) based on cumulative log Gaussian analysis
(see also Heslop et al., 2002). The magnetic components are
characterized by saturation isothermal remanent magnetization
(SIRM), a parameter that is proportional to the content
of the mineral in a sample, the peak field (B1/2 ) at which
half of the SIRM is reached, and the dispersion (DP) of its
corresponding cumulative log-normal distribution (Kruiver
et al., 2001). Magnetic hysteresis loops were measured on small
size rectangular samples (<5 mm in length; max. field = 1.0 T).
Hysteresis loops are corrected for the dia/paramagnetic effect
and normalized by mass. There are four major parameters
obtained: Ms, saturation magnetization; Mrs, saturation
remanent magnetization; Hc, coercive force or coercivity; Hcr,
remanent coercivity (which is determined by operating backfield DC demagnetization up to -1.0 T after IRM acquisitions
had been imparted up to +1.0 T). FORC diagrams were realized
to model magnetic mixtures and magnetostatic interactions.
FORC diagrams were realized to better constrain mixtures and
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Results
Determination of Magnetic Mineralogy
Magnetite and hematite are the main magnetic minerals in
the samples as revealed by temperature dependent magnetic
susceptibility curves (Figure 4) (n = 12). However, the presence
and relative contribution of hematite are difficult to define
on these experiments (they are represented by a second slight
susceptibility decay above 580◦ C) because magnetite has a
much higher intrinsic magnetic susceptibility (Kletetschka et al.,
2000).
Magnetite also dominates the IRM acquisition curves (more
than 80% of total magnetization) (Figure 5C). Magnetite can be
divided into two populations, with variable proportions from one
sample to another, identified by a statistical analysis (Kruiver
et al., 2001). The more abundant magnetite population (called
high coercivity magnetite component = CHMag ) represents
between 43 and 97% of the magnetization (Table S1 in
Supplementary Material). This population is characterized by
a medium coercivity (mean B1/2 = 63.2 mT) and a mean
SIRM between 4.4 × 10−4 and 1.23 × 10−1 A/m (mostly
between 10−3 and 5 × 10−2 A/m). The second magnetite-like
population (medium coercivity magnetite component = CMMag )
has a lower mean coercivity (mean B1/2 = 25.9 mT; Table S1
in Supplementary Material). It can represent up to 47% of the
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Is the Neoproterozoic oxygen burst a supercontinent legacy?
FIGURE 4 | Variation of low-field magnetic susceptibility during a heating-cooling cycle (red and blue lines, respectively).
FIGURE 5 | Magnetic properties of the Biotite granite of Socotra. (A) Linear trend and
samples. The diamonds, squares, and triangles, respectively correspond to the localization in
Figure 3) (B) IRM acquisition curves (linear and Gaussian cumulative treatment, respectively,
appearance with decreasing magnetic grain size. (C) FORC diagrams showing SD and PSD
increasing magnetic interactions with increasing magnetic grain size.
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5
PSD behavior on a Day-plot of Biotite granite
Haggier Mont, Qalansya, and Sherebrub zones (see
showing magnetite dominance and hematite
behavior disappearance to a MD-like behavior and
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Is the Neoproterozoic oxygen burst a supercontinent legacy?
magnetization with SIRM varying between 7 × 10−4 and 5.3 ×
10−2 A/m.
Hematite component (called high coercivity hematite = Chem )
is represented by a higher coercivity, with a mean B1/2 =
490 mT (Table S1 in Supplementary Material). This component
represents on average only 4% of the total magnetization, but it is
not evenly distributed. In fact, hematite-like pattern corresponds
to 1–2% of magnetization in the majority of samples, but for some
samples, like CS6, it represents more than 20% of magnetization
(Table S1 in Supplementary Material).
A low coercivity component (called low coercivity magnetite
component = CLMag – mean B1/2 = 6.4 mT) represents 7% of
the total magnetization on average (Table S1 in Supplementary
Material). This component could be maghemite or coarsegrained magnetite.
but has higher Hcr /Hc ratio. This strongly suggests a binary
mixing solution between two magnetite populations that belongs
to the SD domain for the small one and lies at the base of
the PSD domain and near the MD domain for the larger
one. Magnetostatic interactions can strongly affect the hysteresis
parameters, shifting them toward the SD behavior for large grains
(see Sprowl, 1990; Muxworthy et al., 2003) and therefore change
the trend of a binary mixing solution. However, here we can
discard this effect since few interactions are detected with the
FORC measurements.
The trend on the Day plot (Figure 5A) can be interpreted
in two ways: a mixing between two end-members (SD and MD
grains) or a variability from PSD grains to more MD grains.
We argue here that the hematite and ilmenite lamellae define
subdivisions into magnetite grains, which result in smallerdomain magnetite (<5 µm in average), explaining the PSD
behavior observed. We discuss that the abundance and the
increasing thickness of ilmenite–hematite lamellae result in
tightening of the exsolution treillis. So, this treillis tightening is
consistent with the decreasing of domain size in magnetic grains
from a MD-behavior to a PSD-behavior.
Hematite was detected both by microscopic observations
and by IRM analyses. The presence of hematite has an effect
on hysteresis parameters. Indeed, hematite, even in quite large
grain sizes, has Mrs /Ms -values of 0.5–0.6 and Hcr /Hc -values
of 1.5–2 that mimic those of SD magnetite (Dunlop, 1971;
Dankers, 1977). This corresponds to values of granitic samples
with higher Mrs /Ms . Nevertheless, hematite is known to display
“square” hysteresis curves (Tauxe et al., 1996; Brownlee et al.,
2011). This type of shape and its influence on hysteresis
curves, are not detected in our results. As well, no shapes
characteristic of mixtures like potbellied or wasp-waisted curves
are identified in our samples. The occurrence of hematite in
magnetic parameters (hysteresis curves and FORC diagram in
particular) is observable only from 88% of hematite (Carvallo
et al., 2006) recorded in the total magnetization in magnetic
mixtures with magnetite. However, according to microscopic
observations, content of hematite in the Socotra samples never
reach more than about 20% of the samples. Thus, there is no
surprise that the presence of hematite is not detectable with
these methods in the Socotra samples. Nevertheless, comparison
between the IRM data and the Day-plot help to demonstrate that
the increase in hematite proportion in the total magnetization
is associated with decreasing magnetic grain size of samples. In
addition, it also applies to the lamellae of ilmenite. According
to our microscopic observations, ilmenite having a paramagnetic
behavior, their thickness and their number increases in concert
with the decrease of magnetite grain size.
Grain Sizes and Magnetic Interactions
Hysteresis parameters (Table S2 in Supplementary Material) and
their classical representation on a Day plot (where the ratio
Mrs /Ms is plotted against the ratio Hcr /Hc ) are used to determine
magnetic grain size or to compare relative proportions of two or
three populations (Carter-Stiglitz et al., 2001; Dunlop, 2002a,b;
Heslop and Roberts, 2012). Here, we observed that magnetic
parameters display a trend parallel to SD-MD mixing curves
(Figure 5A), corresponding to a magnetic grain size ranging
between 1 and 10 µm. Hysteresis loops (n = 27) are not
distorted. We observed a thinning of loops with the branch that
straightens up before saturation from high to lower Mrs /Ms ratio.
Inherent ambiguities in interpretation of the Day plot such as
those induced by magnetostatic interactions (Muxworthy et al.,
2003) and by mixtures could be deciphered using first-order
reversal curve (FORC) diagrams (Pike et al., 1999; Carvallo et al.,
2006). FORC diagrams (n = 14) (Figure 5B) obtained for the
Biotite granite display the variability from PSD grain size to
more MD grain size. They also clearly show the presence of two
components for samples lying close to the SD region in the Day
plot. An example is given by CS6 sample that presents a mixture
between two components of relatively low to medium coercivity.
The first component have a PSD-like pattern with a coercivity
peak close to the vertical axis and the second component have a
coercivity peak at about 20 mT, and it corresponds to a SD-like
pattern with contours closing on the diagram. From the highest
to the lower Mrs /Ms ratio, the SD-like component evolves from
sample CS6 (coercivity peak = 20 mT) to sample CS45 (coercivity
peak = 15 mT) and then disappear from sample CS46 (Figure 5).
The PSD-like pattern tends to migrate to a MD-like pattern
(from samples CS46 to CS31). We notice a negligible spreading
parallel to the vertical axis of FORC diagram from sample CS5 to
sample CS31 that indicates that no interactions occurred between
magnetic grains. Moreover, in all FORC diagrams, we notice that
coercivity peaks are slightly displaced above the horizontal axis
that confirms the hypothesis of relative non-interactions between
grains.
What is striking in the log–log Day Plot is the linear
distribution of hysteresis parameters for both Biotite Granites
and Vertical Plutonic Sheets formation. This trend is parallel
to the SD-MD mixing curves as proposed by Dunlop (2002a)
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SEM-light Reflected Microscopy
To establish the relationship between magnetic oxide populations
and their relation to magnetic parameters of the Biotite
granite, we performed microscopic observations on eight thin
sections (Figures 6, 7). Magnetic oxides present in the Biotite
granite of Socotra were found to be associated with two
mineral assemblages: (1) ferromagnesian minerals (biotite and
amphibole) that represent the most common association and
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Is the Neoproterozoic oxygen burst a supercontinent legacy?
<1 and 2 µm) and by internal or external Composites types
(Haggerty, 1991). Rutile lenses and indistinct mottling in ilmenite
lamellae (Haggerty, 1991) were not observed in the Biotite
granite. These elements denote the very high oxidation state of
the Biotite granite.
Discussion
Coexistence of Magnetite and Hematite
The crystallization of magnetic minerals (iron oxides and
sulfides) is strongly dependent on the oxygen fugacity of the
host magma (Haggerty, 1991). Moderate to high oxidation states
correspond to the Fayalite–Magnetite–Quartz (FMQ) buffer of
the present-day mantle. A very high oxidation state would
correspond to the Magnetite-Hematite (MH) buffer, and would
imply the synchronous crystallization of these two iron oxides.
But the coexistence of primary hematite and magnetite in
magmatic rocks is rare; extremely oxidized magmatic rocks being
only found in some Phanerozoic copper porphyries–granitoids
(Sun et al., 2013; Zhang et al., 2013). In Socotra, magnetite
and hematite coexist, and are intertwined in trellis, sandwich,
and composite textures (Figures 6, 7). In this configuration,
hematite divides the large oxide grains (>10 µm) into smaller
ones producing a pseudo-single domain (PSD) behavior for
magnetite (Figure 5). Hematite and magnetite were formed at
the beginning of magma crystallization and ilmenite appeared
after all of them, occurring as inclusions in magmatic biotite
(Figure 7). Both petrographic and magnetic parameters indicate
that hematite is primary, and formed at the early magmatic
evolution of the granite at very high oxygen fugacity and does
not result from hydrothermalism or subsequent alteration.
FIGURE 6 | Photomicrographs under reflected light microscopy of
subhedral crystals of magnetite (Mt) with hematite (He) and ilmenite (Il)
from the Neoproterozoic terrains of Socotra (Oman).
(2) feldspars (perthite and zoned plagioclase). Both associations
are marked by magnetite with both ilmenite and hematite
exsolutions representing the Fe–Ti oxides (sometimes with
some amount of Mn) that dominate the Biotite granite oxide
mineralogy.
Magnetites with exsolutions associated with ferromagnesian
minerals are subhedral to anhedral crystals of large size
(>100 µm). They are disseminated into biotite and amphibole
zones either in isolated grains or in the form of grain clusters
or in the form of large zone of oxides in close association to
the ferromagnesian minerals. Magnetites associated to feldspars
are finer (mean size about 30 µm). Their crystal’s shapes range
from euhedral to subhedral and seldom anhedral. They are
disseminated into zonated plagioclase and perthites. Individual
ilmenite grains with variable Mn content, mainly associated with
feldspar grains, can be found in few samples.
Microtextures of Fe–Ti oxide minerals in Biotite granite
were observed closely with SEM and light-reflected microscopy.
According to Buddington and Lindsley (1964) and Haggerty
(1991), degrees of oxidation and diffusion result in series of
exsolutions in titanomagnetite varying in morphology, size
and in abundance. Using their terminology, ilmenite–hematite
intergrowths occur as Treillis, Sandwich and Composite types
exsolution lamellae in the host Ti-poor magnetite from the Biotite
granite. Following this classification, oxide grains observed in
Biotite granite samples can be classified between C2 stage
and C4 stage of oxidation. It should be noted that C1 stage,
which corresponds to homogeneous Ti-rich magnetite has not
been identified in our samples of the Biotite granite. The
oxidation index of all samples always reaches the C4 stage,
which corresponds to the beginning of titanomagnetite-ilmenite
intergrowths oxidation. It is superimposed most often into
C2 and C3 stages previously formed. This stage results in
formation of hematite in Ti-poor magnetite host and normally
in formation of rutile in ilmenite lamellae. Textural forms of
hematite are characterized by Trellis types (lamellae between
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Implications for the NOE
The intergrowth of hematite and magnetite observed in the
Socotra granites indicates a very high oxygen fugacity (fO2 ) for
the magmas at this margin of Rodinia, around the MH buffer
(1FMQ between +4 and +5). Rocks in subduction settings are
known to be oxidized (Parkinson and Arculus, 1999; Evans et al.,
2012), with fO2 -values of most subduction-related igneous rocks
at 1FMQ +2. The mechanisms invoked to explain their high
oxidation are H2 O incorporation from the subducting plate and
subduction of oxidized materials like carbonates, aseismic ridges
(Sun et al., 2013) among others. Yet, these values remain well
below the values found for Socotra Island granites. Interestingly,
even if the database remains up to now too sparse and more
detailed investigations on the fO2 for other Neoproterozoic
magmatic rocks are needed, other examples of highly oxidized
igneous rocks of similar age have been reported for the Eastern
Desert of Egypt (Ahmed, 2013; Khedr and Arai, 2013), and the
Indian craton granites of Malani suites (Ashwal et al., 2013),
all of them emplaced across the margin of Rodinia (Figure 2).
Another occurrence may be suspected in South China (Liu et al.,
2012; Zhang et al., 2013). The high apparent fO2 at the Arabian–
Nubian shield in Egypt and Socotra, NW Indian, and perhaps in
South China, imply a significant reservoir of oxidized magma in
the upper mantle between at least between 800 and 750 Ma on the
magmatic arcs at the western hinge of Rodinia.
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Is the Neoproterozoic oxygen burst a supercontinent legacy?
FIGURE 7 | Photomicrographs of magnetite. (A) Photomicrographs of subhedral crystals of magnetite (black minerals) trapped in a poikiloblast crystal of biotite
(brown mineral) from the Neoproterozoic terrains of Socotra (Oman). (B) Recognition of treillis-like shaped hematite within the crystal of magnetite under reflected light
microscopy. (C) Back-scattered electron imaging on the internal structure of oxide, using a scanning electron microscope (SEM), reveals multiple grains of
magnetite-hematite separated by elongated crystals of ilmenite. (D) Oriented needles or patches of ilmenite outline the contact with biotite. (E,F) Interpreted
chronology of magmatic events depicted from microscopic and SEM observations: (1) treillis-like shaped hematite developed early along cleavage planes of
magnetite, due to magmatic oxy-exsolution processes; (2) magnetite was further fractured and divided into several grains; (3) Cracks were subsequently filled with
ilmenite growing in equilibrium with the large magmatic biotite.
O2 consumed by reaction with reduced volcanic gases (Fv),
reduced metamorphic gases (Fm), and reduced material on the
continents Fw (Catling and Claire, 2005). Following Claire et al.
(2006), we consider Fv and Fm to be around 1.5 Tmol.yr−1 of
equivalent O2 consumption each. In this way, a first estimation
on the impact of oxidized to very oxidized emissions to Fv
can be provided. For that, we used the estimation given by
Gerlach (2011) where the proportion of emitted gases (CO2 )
from arc volcanoes nowadays is at 30–34% of the total amount
Now we can estimate the impact of volcanic oxidized gases
from Rodinian magmatic arcs on the atmospheric composition.
In steady state, the O2 budget can be described following Catling
and Claire (2005) and Claire et al. (2006) (Equation 1):
Fe + Fb = Fv + Fm + Fw ,
(1)
where source fluxes correspond, respectively, to the escape of
hydrogen to the space (Fe) and the total O2 released from
organic matter and pyrite burial (Fb). The sink fluxes are the
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Macouin et al.
Is the Neoproterozoic oxygen burst a supercontinent legacy?
of gases on Earth. Using the conservative estimate of 30%, that
would represent 0.45 Tmol.yr−1 of the 1.5 Tmol.yr−1 emitted by
volcanic fluxes and considering a minimum time of 50 Ma for the
duration of oxidized emissions in subduction zones, this would
correspond to a net flux of O2 of at least 2.25 × 107 Tmol.
The rough estimation obtained above corresponds to oxygen
concentrations of at around 0.5 PAL (Lasaga and Ohmoto, 2002).
Such a burst of oxygen, due to a decrease in one of the most
important sinks of O2 during the lifetime of Rodinia (of at least
50 mol.yr) would be large enough to increase significantly the
oxygen content of the atmosphere (Laakso and Schrag, 2014).
Our findings have two additional implications. First, they imply
that the NOE may have started earlier than generally thought
(Sahoo et al., 2012), and before the Neoproterozoic low-latitude
glaciations, in line with most recent findings (Sahoo et al., 2012;
Baldwin et al., 2013; Ader et al., 2014). Second, by starting
atmospheric oxygenation earlier (Figure 1A), our conceptual
model also provides an explanation for the Neoproterozoic
glaciations themselves. If the NOE occurred prior to the first
glaciation, it would have likely resulted from the oxidation of
a methane-rich atmosphere (Pavlov et al., 2003) as previously
evocated by Claire et al. (2006). In this case, the drop in methane
and ethane (which are both greenhouse gases) would have caused
the drop in surface temperature promoting the onset of severe
glacial episodes at the end of the Proterozoic.
Acknowledgments
The study is supported by the Syster and Marges programs of
the INSU-CNRS and by the OMP-AO1 program. MM benefited
from three months of invitation to stay at IAG, Brazil supported
by grant 2013/08862-1, São Paulo Research Foundation
(FAPESP).
Supplementary Material
The Supplementary Material for this article can be found
online at: http://journal.frontiersin.org/article/10.3389/feart.
2015.00044
Table S1 | Isothermal remanent magnetization (IRM) acquisition data
showing the repartition of magnetic components for each rock formation
of Socotra.
Table S2 | Hysteresis parameters, susceptibility and remanent
magnetization of each selected rocks of Socotra basement. Hcr is the
coercivity of remanence, Hc the coercive force, Mr the saturation remanence, and
Ms the saturation magnetization.
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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2015 Macouin, Roques, Rousse, Ganne, Denèle and Trindade. This
is an open-access article distributed under the terms of the Creative Commons
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